Some hospital beds are equipped with wheels to enable transport of patients between individual workplaces of the hospital. In the corners of the bottom frame of such a bed there are four travelling wheels that are freely turning around the vertical and horizontal axis. Such a design makes it possible to move the bed easily in all directions, but during the movements along hospital corridors it is difficult to keep the straight direction. This is why some beds are equipped with the fifth, so called guiding wheel that can be locked in the straight direction, so the bed keeps the straight direction.
The guiding wheel is usually positioned in the middle of the undercarriage, it is spring loaded and pressed to the floor with the force of approx. 200 to 500N. The control, i.e. locking of the guiding wheel in the straight direction may be either separate or derived from the control of the main wheels, or it may be self-locking.
A disadvantage of structures with separate control where the wheel is lowered onto the floor in such a way that the control force must overcome the force of the pressing spring is that the control force is relatively high while the accumulated energy of the compressed spring causes impacts during the control as well as noise when the wheel hits the floor.
In structures with a horizontal rotation axis combined with a vertical axis and locking control derived from the control of main wheels the locking in the straight direction is “pre-selected”, but it is only activated when the bed starts moving after the part with the vertical axis turns to the straight direction due to the turning of the guiding wheel and the lock engages. An advantage is a low control force. A disadvantage in this case is delayed locking as the wheel is not locked in the straight direction until the corresponding parts of the locking mechanism are turned towards each other.
In structures with a horizontal rotation axis combined with a vertical axis and self-locking of the guiding wheel in the straight direction e.g. a spring-loaded ball is used against which a groove is created in the required place where the ball may fit into to prevent free turning of the guiding wheel around the vertical axis. An advantage is that you do not need any other control since the wheel is locked automatically in the straight direction as soon as the ball fits into the corresponding groove. A disadvantage is that if it is necessary to move the bed in another than straight direction, you must exert an increased pressure on the bed to make the spring-loaded ball get out of the groove to deactivate the locking of the guiding wheel while the straight direction is locked again when during the turning of the wheel around the vertical axis the ball gets to the groove again. This is also related to the risk of spontaneous unlocking of the guiding wheel when passing through a bed due to an excessive side force.
In cheaper version of beds the fifth guiding wheel is not installed at all and locking in the straight direction is achieved by locking of one of the four main travelling wheels. However, this solution requires higher skills of the personnel with regard to moving the bed and at the same time larger handling areas for turning (wider corridors) since the free end of the bed gets considerably out of direction.